The present invention relates to a circuit arrangement for monitoring or testing the connecting lines of a switch or sensor open in the normal condition, with a first resistance connected in series with the same which, together with the switch or sensor and a second resistance connected in parallel to both, forms a parallel circuit, whose first connecting line is connected to ground and whose second connecting line is connected, on the one hand, by way of a third resistance to a voltage supply and, on the other hand, with a voltage discriminator unit.
Such a circuit arrangement is used already for monitoring or testing the functioning of the connecting lines to a sensor of an air bag control apparatus of a motor vehicle. As in this known circuit arrangement, the output voltage of the volage supply has been selected smaller than the battery voltage of the motor vehicle, the connecting line of the sensor connected to the voltage supply can be tested or monitored as regards a short circuit to ground, a short circuit to the battery voltage as well as a line interruption. Furthermore, the connecting line of the sensor connected with the ground potential can be monitored with respect to a short circuit to the battery voltage as well as a line interruption. These monitoring or testing possibilities of the connecting lines of the sensor can be traced back to the fact that a defined voltage level exists at the voltage discriminator unit both in the two shifting conditions of the sensor with intact connecting lines as also with defective connecting lines corresponding to the respective line defect. Thus, with intact connecting lines and with an open sensor, a voltage exists at the voltage discriminator unit which is determined by the voltage divider formed by the second and third ohmic resistance. Furthermore, with a closed sensor, a voltage exists at the voltage discriminator unit which is determined by the voltage divider formed by the first, second and third ohmic resistance. Of these two voltages which occur at the voltage discriminator unit with intact connecting lines to the sensor, the voltages differ which occur at the voltage discriminator unit in case of the aforementioned defects. Thus, for example, with a short circuit of the connecting line of the sensor connected to the voltage supply with respect to the battery voltage, the battery voltage exists at the voltage discriminator unit whereas in case of a short circuit with respect to ground potential, the ground potential exists at the voltage discriminator unit and in case of a line interruption, the output voltage of the voltage supply exists at the voltage discriminator unit.
This known circuit arrangement only entails the disadvantage that with an operationally ready sensor, a diagnostic current always flows also in the open position thereof by way of the second and third ohmic resistance. This has as a consequence that the known circuit arrangement can be used in the motor vehicle industry only with such sensors, respectively, switches which are not connected to the voltage with a turned-off ignition switch because otherwise the battery will be discharged during the standstill of the motor vehicle, as a result of which after a longer standstill period of the motor vehicle the latter can no longer be started. It is therefore the obect of the present invention to so further develop a circuit arrangement of the aforementioned type that it can also be utilized for monitoring the connecting lines of switches or sensors which remain connected to voltage also with a turned-off ignition switch.
The underlying problems are solved according to the present invention in that the second ohmic resistance is adapted to be diconnected.
As the second ohmic resistance is not necessary for the functioning ability of the switch or of the sensor, but is required only in order to be able to monitor or test with an open switch or sensor the connecting line thereof connected to the voltage supply as regards a line interruption, it can be disconnected for reasons of current economy during a predetermined time period, for example, during the standstill of a motor vehicle, during which a monitoring or testing of the connecting lines of the switch or of the sensor is not required.
This takes place advantageously according to a further feature of the present invention in that a Zener diode is provided in the current branch containing the second ohmic resistance of the parallel circuit formed by the first and second ohmic resistance as well as by the switch or sensor, and in that the voltage supply is adapted to be switched from a greater to a smaller output voltage in comparison to the breakdown voltage of the Zener diode. With the use of such a circuit arrangement in a motor vehicle, the voltage supply is preferably so switched that it produces an output voltage during the driving operation of the motor vehicle which is larger than the breakdown voltage of the Zener diode with the consequence that the Zener diode is rendered conducting in the blocking direction. In this condition, a diagnostic current can flow by way of the second or third ohmic resistance with an open switch or sensor which is required for the determination of a line interruption of the current-conducting line of the switch or sensor. For disconnecting the second ohmic resistance during the motor vehicle standstill, the voltage supply is switched to an output voltage which is smaller than the breakdown voltage of the Zener diode. As a result of this measure, the Zener diode is switched to be non-conducting in the blocking direction, with the consequence that no diagnostic current can flow any longer through the current branch containing the second resistance. It is achieved thereby that the circuit arrangement according to the present invention represents no current load during the motor vehicle standstill with the exception of the negligible leakage current of the Zener diode.
In a second, alternative possibility for disconnecting the second ohmic resistance, an electronic switch is provided in the current branch containing the second ohmic resistance. With the use of this circuit arrangement in the motor vehicle, the control input of the electronic switch is preferably so connected to the ignition circuit of the motor vehicle that the electronic switch is switched to be conductive when the ignition switch is closed and is rendered non-conductive when the ignition switch is opened. It is achieved thereby that only with a closed ignition switch, i.e., as a rule in the driving operation of the motor vehicle, when the generator or alternator of the motor vehicle continuously recharges the battery, a diagnostic current can flow through the second ohmic resistance which is required exclusively for the monitoring or testing of the connecting lines of the switch or of the sensor but is not required for the functioning of the switch or sensor. Owing to the fact that with an open ignition switch, no diagnostic current can flow through the second ohmic resistance not required for the functioning of the switch or sensor, a discharge of the battery during a motor vehicle standstill is avoided.